Edible oils are composed of a diversity of triglyceride molecules (triester molecules consisting of glycerin and any three of a variety of fatty acids). The fatty acids typically contain twelve, fourteen, sixteen or eighteen carbon atoms, although some longer and some shorter fatty acids can also be present. With the exception of the tropical oils such as palm and coconut oil, the majority of fatty acids in most plant- and animal-derived oils of commercial significance contain eighteen carbon atoms (C18). Each of those C18 fatty acids can contain zero, one, two or three unsaturated carbon-to-carbon bonds and are referred to as saturated (18:0), monounsaturated (18:1), di- or doubly-unsaturated (18:2) or tri- or triply-unsaturated (18:3), respectively. Fatty acids that contain two, three or more unsaturated bonds are referred to as polyunsaturated. The shorter fatty acids are rarely present in unsaturated form. Fatty acids that contain three or more unsaturated bonds are normally unstable and oxidize readily in air or when exposed to light.
Polyunsaturated fatty acids are not metabolically synthesized in mammals, and are therefore termed “essential fatty acids” as nutrients in the mammalian diet. The most common polyunsaturated fatty acids include the so-called “omega-6” fatty acids [e.g., the 18 carbon molecule, linoleic acid (18:2), that has two carbon-carbon unsaturated chemical bonds] and the so-called “omega-3” fatty acids, [e.g., the 18 carbon molecule, alpha-linolenic acid (18:3), that has three carbon-carbon unsaturated chemical bonds], as well as longer chain omega-3 molecules such as docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA) that have greater numbers of carbon-carbon unsaturated chemical bonds, and that are abundant in fish oils.
As the number of carbon-carbon unsaturated double bonds in a fatty acid molecule increases, the oxidative stability of triglyceride fats and oils containing these fatty acids decreases. Off-flavor development and rancidity typically accompany oxidation of omega-6 and omega-3 fatty acids. For example, the omega-3 fatty acids in conventional canola oil (containing approximately 9 percent by weight alpha-linolenic acid) are susceptible to oxidation. Consequently, this oil is not recommended as commercial frying oil because heating accelerates the process of fatty acid oxidation. Oils such as flaxseed oil that contain a much higher concentration of alpha-linolenic acid (57 percent) are considerably more susceptible to oxidation than canola oil and must be refrigerated and protected from light to prevent the development of off flavors. Oils such as menhaden fish oil contain the longer chain omega-3 fatty acids, e.g., DHA and EPA, are also unstable with exposure to oxygen and light, and can rapidly develop off-flavors.
Accordingly, it is challenging to develop new strategies for stabilizing fats and fat-containing foods carrying omega-3 fatty acids. Of the omega-3 fatty acids, alpha-linolenic acid is not as unstable as docosahexaenoic acid (DHA) or eicosapentaenoic acid (EPA), and is therefore a good candidate for including as a supplement in foods. The problem of omega-3 instability and off-flavor development in food is widely appreciated, and is evident in the following patent literature.
Kantor et al., in U.S. Pat. No. 4,895,725, describe foods that contain vegetable oils capable of masking the odor and taste of omega-3 fatty acids in fish oil, providing that the fish oil has been first microencapsulated within gelatin or gelatin-acacia gum microcapsules.
Akahoshi et al., in U.S. Pat. No. 6,025,008, describe yoghurt containing the omega-3 fatty acids, DHA or EPA, in which sweet substances; i.e., reducing sugars, together with oxygen-blocking packaging can prevent the development of fishy odor in the yoghurt.
Schroeder et al., in U.S. Pat. Nos. 4,913,921, No. 4,963,380, and No. 5,084,294, teach addition of fructose to fish oil-supplemented food products to help control oxidative rancidity owing to the presence of omega-3 fatty acids.
Antrim et al., in U.S. Pat. No. 4,963,385, teach adding sugar, sugar alcohols and metal ion chelators to aqueous food emulsions containing a fish oil to prevent rancidity.
Marquez et al., in U.S. Pat. No. 6,428,461 B1, describe a method for reducing oxidative rancidity of polyunsaturated lipids containing omega-3 fatty acids by combining a number of different polyamine compounds with the lipids.
Bracco et al., in U.S. Pat. No. 5,518,753, describe a triglyceride mixture or a food composition-containing a triglyceride mixture in which the fatty acid composition includes 50-70 percent by weight monounsaturated oleic acid, up to 10 percent by weight saturated fatty acids and from 30 to 40 percent polyunsaturated fatty acids. The polyunsaturated fatty acids include 20-35 percent linoleic acid and several omega-3 fatty acids including 1.5-4 percent C18:3,n-3 (alpha-linolenic acid), 0.1-0.5 percent C18:4,n-3, 0.2-1 percent C20:5,n-3 (EPA) and 0.1-0.8 percent C22:6,n-3 (DHA). Although the moderately elevated level of oleic acid can contribute to oxidative stability, it has been found that the substantial concentration of linoleic acid (20-35 percent by weight) in the composition decreases oxidative stability of omega-3 fatty acids. Therefore, the composition of Bracco et al. is not useful in the present invention.
An example of oxidative stabilization of an omega-6 fatty acid may be found in the data of Sundram et al. in U.S. Pat. No. 5,874,117. That patent describes corn oil and its linoleic acid content that is surprisingly well stabilized against oxidation by dilution into palm oil that is rich in saturated fatty acids.
In recent years, plant breeding has been used to improve the oxidative stability of soybean oil and canola oil by reducing their substantial levels of omega-3 alpha-linolenic acid (i.e., 6-10 percent) as well as omega-6 linoleic acid. Similarly, within the past ten years, plant breeding has been used with peanuts to produce peanut oils that contain elevated levels of oleic acid and greatly reduced levels of linoleic acid.
For example, conventional peanuts contain approximately 46 percent by weight oleic acid and approximately 32 percent by weight linoleic acid, (and approximately 17 percent by weight mixed saturated fatty acids). However, the peanut oils resulting from plant breeding described by Moore in U.S. Pat. No. 5,945,578 contains between 80 and 85 percent oleic acid and only 1.5 to 2.5 percent linoleic acid. The peanut oil described by Knauft et al. in U.S. Pat. No. 6,121,472 contains between 74 and 84 percent oleic acid and only 2 to 8 percent linoleic acid, whereas the peanut oil of Horn, et al. in U.S. Pat. No. 6,214,405 contains between 78 and 82 percent oleic acid and only 2.8 to 4.9 percent linoleic acid.
Thus, the linoleic acid content of these varieties of peanuts has been reduced approximately 10-20-fold, whereas the oleic acid content has been nearly doubled (i.e., a 20-fold increase in the ratio of oleic:linoleic acid) compared to conventional peanut varieties. The improved oxidative stability of these peanut oils resulting from the higher level of oleic acid and lower level of linoleic acid has been recognized by the inventors of the above patents.
Similarly, breeding has altered the content of fatty acids esterified in canola and other oils. Thus, Corbett (2002) PBI Bulletin 1:1-4, reports a high oleic canola available commercially under the designation Natreon™ contains 75 percent oleic, 14 percent linoleic and 3 percent linolenic acids, with the remaining saturated fatty acids comprising less than 7 percent. A so-called low linolenic oil was said to contain 65 percent oleic, 22 percent linoleic and 4 percent linolenic acids. A commodity canola oil was said to contain 60 percent oleic, 20 percent linoleic and 10 percent linolenic acids. High oleic acid sunflower oil was said to contain less than one percent linolenic acid, as do high oleic acid safflower and olive oils, whereas high oleic acid soybean oil contains 3 percent linolenic acid.
After the effort of selecting and breeding varieties of soybeans, canola, and peanuts whose expressed specialty oils contain reduced levels of linoleic acid and exhibit improved oxidative stability, it would seem counterproductive and counterintuitive to add back an edible oil rich in polyunsaturated fatty acids because such addition would presumably reduce the oxidative stability of the specialty oil. Indeed, the literature does not suggest the addition of omega-3 polyunsaturated fatty acids to peanut butter, peanut oils or other comminuted peanut-containing products that have been produced from the special varieties of peanuts described above by Moore, Knauft et al. or Horn et al. that are distinguished by their low level, e.g., generally between 2 and 8 percent, of the omega-6 fatty acid, linoleic acid. The disclosure that follows illustrates an oxidative stability advantage that is obtained by the addition of an oil rich in omega-3 fatty acids to products of one of the above-described special varieties of oil.